The sawing of large metal objects or workpieces such as ingots, heavy plates and the like must be routinely and frequently performed in large scale industrial operations. Although the operation normally requires no more than the slitting of the workpiece, the actual cutting task is laborious, slow and time-consuming, and consumes cutting tools, normally saw blades, at a rapid rate. Consequently, the task is relatively expensive.
In the past, best cutting performance was obtained with abrasive saws. Such saws employ abrasive, e.g., hard faced, diamond-studded, etc., saw blades or discs which are advanced into the workpiece to cut a kerf therein. With increasing workpiece size, and they can have cross-sectional dimensions of up to 65 cm. and more, the construction of the saw blade and of the actual saw becomes more and more complicated and expensive. Yet, maximum cutting rates for such saws presently do not exceed about 775 cm.sup.2 /min. of the workpiece cross-section when it is constructed of steel and about 2 to 21/2 times that rate when it is constructed of an aluminous material. At such cutting rates, the abrasive discs are subjected to extreme stresses, are rapidly worn down and represent a constant danger to persons and property in the vicinity of the saw because of possible blade ruptures caused by the high rpm at which such blades must be operated.
It is also known to flame-cut oxidizable materials, primarily steel, with acetylene-oxygen burners. This process is very slow, wastes a great deal of metal, and is not suitable for making deep cuts because there is no good control over the location and orientation of the cutting flame. Because of the stated drawbacks of this system it is usually limited for cutting plate, bars and the like which have a thickness no more and preferably less than about 10 cm.
It has also been proposed to employ so-called arc saws to cut metallic workpieces. According to this approach, a cut is formed by electrically melting a kerf into the workpiece, normally by providing a rotating blade and applying an electric potential between the blade and the workpiece. The blade is then moved towards the workpiece until it makes contact to initiate an electric current flow. The current flow melts the material which is removed from the kerf by the rotating blade.
Although this last mentioned approach appears promising on its face, it has not found acceptance; in fact, it is doubtful that it has ever been employed in a commercial application. The apparent failure of the electric arc sawing probably results from the fact that arc sawing as such has never been understood. A review of the relevant prior art indicates an overall concensus that in order to arc saw, one requires no more than a suitable, normally round saw blade and a "suitable" electric power source which applies a potential between the object to be cut and the blade and which generates a "relatively high" current. U.S. Pat. Nos. 1,701,919; 2,015,415; 2,059,236; 2,355;838; and Re 20,035 are exemplary of the present state of the prior art as known to the inventors.
In all instances, the prior art patents are primarily concerned with the construction of the hardware for an arc saw, to wit the manner in which the saw is mounted, driven and advanced relative to the workpiece; the location of spindles, handles and switches; and the actual configuration of the saw blades, it having been thought that the blade should have conventional saw teeth or variously oriented, shaped and sized serrations to enhance the sawing process. In some cases, such saw blades have been provided with a means for injecting an electrolyte into the kerf during the cutting operation. See, e.g., U.S. Pat. No. 3,338,808, No. 3,663,787 and Russian Pat. No. 180,275.
The prior art is conspicuously silent as to the actual operation of the saw, the voltage and current requirements, the quality of the kerf that is cut into the workpiece and the like. These latter aspects, however, primarily determine whether or not a cut can be made at all and if so, whether or not it has the desired quality; and whether the saw, and especially the saw blade is capable of withstanding the intended use. In regard to these aspects the prior art provides no clue whatsoever. As a result, one intending to build an arc saw in accordance with the prior art is faced with developing his own power supply, being told no more than that it should yield a "relatively high current" though it is apparent on its face that a high current for one application may be an insufficient current for another operation.
The inventors of the present invention, during actual experimentations as well as during theoretical considerations of the needs of arc sawing quickly determined that arc saws constructed in accordance with the prior art simply were not operable. To be more precise, the prior art fails to teach or suggest how an arc saw can be efficiently and effectively built and operated. Problems encountered with arc saws constructed along the lines suggested in the prior art included an inability to sustain the cutting action of the saw; erratic and highly uneven and, therefore, low quality cuts; rapid wear and tear of the saw blades; unexpected and unexplainable current variations and surges during cutting; difficulties in controlling such cutting parameters as the cutting voltage, current and speed, the observation of a relative non-responsiveness of the cutting operation to changes in the applied voltage; and the like.
The inventors of the present invention have identified two problems which must be overcome by any successful electric arc saw. First, the cutting debris which accumulates in the kerf as the blade advances through the material should be cleared to the extent possible before beginning a second cut along the same kerf. Such debris can interfere with succeeding cuts and possibly damage the saw blade. Second, both the saw blade and the workpiece must be cooled to prevent overheating. If the saw blade is not adequately cooled, it has been found that the sawblade will deform. Even a very small lateral deformation in the blade causes the radial face of the blade to approach the workpiece, resulting in arcing and shorting between the radial face of the blade and the workpiece. Such arcing can cause unwanted pitting along the surfaces of the kerf and also damage the saw blade.
In addition to problems relating to the cutting per se, prior art saws were for most applications environmentally simply unacceptable. Unless the cutting is performed in a totally isolated atmosphere, the electric arcing that takes place during the arc sawing operation is so noisy that it is beyond human tolerance and would, for example, fail to meet present day industrial health and safety regulations. Additionally, prior art arc sawing methods develop great amounts of dust and flying debris, cause sight impairing electric sparks, and generate such a high degree of ultraviolet radiation as to require special shielding to prevent operating personnel from suffering skin burns.
It is, of course, not possible to speculate and isolate the one or more exact reasons which prevented arc sawing from becoming a comercially feasible cutting method. It is reasonable to assume, however, that the many drawbacks discussed above combined with the lack of control that could be exercised over the cutting operation and the generally low quality cuts that were made combined to spell the demise of arc sawing on a commercial scale.